1. Field of the Invention
The present disclosure relates to controlling PAPR. Although the present disclosure is suitable for a wide scope of applications, it is particularly suitable for decreasing PAPR characteristics in an orthogonal frequency division multiplexing (hereinafter abbreviated OFDM) system.
2. Discussion of the Related Art
Generally, in the OFDM system, signal processing is performed on a channel tending to have frequency-selective fading in a frequency domain to bring about flat fading. So, the OFDM system enables more efficient communications. Due to this advantage, the OFDM is widely adopted by wireless communication systems.
Meanwhile, the OFDM system has a problem of a peak to average power ratio (hereinafter abbreviated PAPR). If the PAPR is big, a power amplifier having a large linear interval to amplify a signal corresponding to a peak power is needed. Yet, a product cost for manufacturing the power amplifier having the large linear interval is too high. In case that a power amplifier has a small linear interval, a signal amplified in a non-linear interval gets distorted.
To decreasing the PAPR, various methods have been proposed. As an example of the methods, variants of OFDM like SC-FDMA, offset DFT-SOFDM and precoded DFT-S-OFDM are provided. In these methods of modifying OFDM signal generation, PAPR characteristics of a transmission signal is enhanced in a manner of spreading a transmission data vector by DFT before mapping data to subcarriers in a frequency domain and then mapping the data to the subcarriers. In the methods of modifying OFDM signal generation, it is in common that a signal is spread by DFT. In particular, since the data signal mapped by the subcarriers is performed by IDFT at a final transmission step, powers of signals having peak powers are cancelled out to reduce a power variation of the final transmission signal.
FIG. 1 is a block diagram of an example of an OFDM signal generator supporting SC-FDMA,
Referring to FIG. 1, a serial-to-parallel converting unit 11 converts a data symbol inputted in series to a parallel signal. A signal spreading unit 12 performs a dispreading on the paralleled data symbol in a frequency domain using Discrete Fourier Transform (hereinafter abbreviated DFT) before generating an OFDM signal. Equation 1 indicates a method of dispreading a parallel signal using NbsNb DFT matrix.sF=FNb×Nbsx  [Equation 1]
In Equation 1, ‘N’ indicates the number of subcarriers provided to an OFDM signal, ‘sx’ indicates a data symbol vector, ‘sF’ indicates a vector of which data is spread in a frequency domain, and ‘sTx’ indicates an OFDM symbol vector transmitted in a time domain. Moreover, ‘FNb×Nb’ is a DFD matrix of ‘NbsNb’ used in dispreading a data symbol.
A subcarrier mapping unit 13 maps the spread vector sF to subcarrier using a subcarrier allocating pattern. An Inverse Discrete Fourier Transform (hereinafter abbreviated IDFT) unit 14 transforms the signal mapped to the subcarrier into a signal in a time domain. Equation 2 represents Inverse Discrete Fourier Transform.sTx=FN×N−1sF   [Equation 2]
In Equation 2, FN×N is NbsNb DFT matrix used to transform a signal in a frequency domain to a signal in a time domain and sTx is a signal generated in a time domain by IDFT, A parallel-to-serial converting unit 15 converts the parallel signal converted in a time domain to a serial signal. A cyclic prefix inserting unit 16 inserts a cyclic prefix in a signal to avoid interference between OFDM symbols and then transmits the signal.
The offset DFT-S-OFDM system improves the PAPR performance of the SC-FDMA. Yet, the offset DFT-S-OFDM system requires a considerable load of calculations to make its implementation complicated. In particular, in the process of performing DFT dispreading, DFT matrix is applied in a manner of separating an input symbol into a real part and an imaginary part. So, the calculation load increases to make the corresponding implementation more complicated. Hence, the demand for a system, of which implementation is simpler than the offset DFT-S-OFDM system with PAPR performance better than that of the offset DFT-S-OFDM system, rises.